Clark had his best years with the Detroit Tigers (1995–2001), but also played with five other teams during a 15-year career that ended in 2009. He was a switch hitter, and threw right-handed. He was third in Rookie of the Year voting in 1996, and was an All Star in 2001.

Clark was a union representative while he was a player, and after retiring he joined the staff of the MLBPA in 2010.[1] He served as deputy executive director and acting executive director of the union before he was appointed executive director in December 2013, upon the death of Michael Weiner.[1] Clark is the first former player to be executive director of the MLB players' union.[1]

He was third in Rookie of the Year voting in 1996, when he hit .250 with 27 home runs.

His most productive seasons were 1997, with 32 homers and 117 RBIs (though he made 10 errors at first base), 1998, with 34 homers and 103 RBIs (though he made 13 errors at first), and 1999, with 31 home runs and 99 RBIs (though he made 10 errors at first).

Signed as a bench player, Clark filled in for the New York Yankees in 2004 after Jason Giambi was forced out of the lineup because of an injury. Though he was replaced as the main first baseman by John Olerud late in the season, he still had a few memorable performances.

Clark signed with the Arizona Diamondbacks after the 2004 season. In 2005, he enjoyed success with the D-Backs. In a limited role (349 at bats), he hit .307, belted 30 home runs, and knocked in 87 runs.[6]

In 2006, Clark was injured for most of the season. Although he tried to play through a shoulder injury that required significant surgery to repair, he batted a career-low .197, with a career-low .279 on-base percentage, in 132 at-bats. He struggled especially against right-handers, batting .125 against them.

After the season, his contract was up and on February 10, 2008, Clark agreed to a one-year contract worth $900,000 with the San Diego Padres.[7] On July 17, 2008, he was traded back to the Arizona Diamondbacks for minor league pitcher Evan Scribner.[8] In order to complete the trade, Clark waived a clause under his contract with the Padres pursuant to which he was to receive $500,000 from the Padres if traded.[8]

In 2008, between the two teams, Clark batted .225 with a .318 slugging percentage. Clark struck out more than 1⁄3 of the time, with 55 strikeouts in 151 at-bats. He struggled especially against right-handers, batting .198 against them.

Clark filed for free agency after the 2008 season. On January 2, 2009, he signed a one-year deal worth $800,000 to remain with the Diamondbacks.[9]

Clark had a startling good performance on Opening Day 2009, hitting 2 home runs to lead the D-Backs to a victory over the Colorado Rockies; fellow switch-hitting teammate Felipe López also homered from both sides of the plate in the same game, making them the first teammates to do so on an Opening Day.[10]

Clark slumped badly thereafter, however, as in his next 18 at-bats he only managed to eke out a single. As of May 6 he was batting .179, and had struck out in more than half his at bats.[11] That day Clark was placed on the 15-day disabled list for a lingering wrist ligament injury, and Whitesell, who was hitting .356 for the Reno Aces with a .477 on-base percentage, was called up to the Diamondbacks to take his place.[12][13][14] Clark suffered the injury during spring training, and re-aggravated it in late April, leaving him unable to swing comfortably from the left side. It was anticipated that the injury could require more than 15 days to heal.[15] On June 19 Clark came off the disabled list and returned to Arizona (after a rehab assignment at Reno in which he batted .160, and during which he turned 37), and Whitesell was optioned back to Reno (after batting .300 with a .447 on-base percentage in his second stint with the team).[16][17] In his first game back with the team, Clark went 0-3 with 2 strikeouts to bring his batting average down to .161, with strikeouts in 55% of his at bats for the season.

Clark struggled on defense as well, as on June 21 in his second game back he dropped a throw to him at first base with two outs in the ninth, allowing the winning run to score for Seattle.[18] The play left players and managers on both sides stunned and speechless.[18] "It's a miserable ending to a rough road trip", manager A. J. Hinch said.[19] His resulting .973 fielding percentage was last among major league first basemen who had played 60 or more innings.[20]

On July 12, 2009, the Diamondbacks released Clark, who was hitting .182 with four home runs and 11 RBIs. They replaced him with Whitesell. Clark said he would continue to work out the next few weeks in the event an opportunity might arise with another team, and that if he didn't land with another team he'd consider broadcasting and coaching, perhaps with the Diamondbacks. Diamondbacks General Manager Josh Byrnes expressed an interest in keeping him with the organization, and Clark said he "would welcome the opportunity."[21]

Clark played in four post-season series through 2008, two each for the Yankees and the Diamondbacks. In aggregate, he batted .135, with a .158 on-base percentage and a .189 slugging percentage, and drove in one run in 37 at-bats.[22]